Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (“SPE”) membrane or proton exchange membrane (“PEM”) to provide ion transport between the anode and cathode.
In proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel and oxidant to disperse over the surface of the membrane facing the fuel- and oxidant-supply electrodes, respectively. Each electrode has finely divided catalyst particles (for example, platinum particles) supported on carbon particles to promote oxidation of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water which is discharged from the cell. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”) which, in turn, are sandwiched between a pair of non-porous, electrically conductive elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cell stacks in order to provide high levels of electrical power.
High surface area carbon black is often used as a support for fuel cell catalysts. High surface area carbon black often contains large quantities of internal micropores (<4 nm) in their constituent particles. Pt nanoparticles deposited in these micropores can have restricted access to reactants and show poor activity. Studies have shown that up to 80% of all Pt particles are deposited inside the micropores. Opening up these micropores to better expose the Pt particles should improve the high power performance of the catalyst. As used herein, the terms “micropores” and “pores” are used interchangeably, not to be mistaken with mesopores (pores of 5-15 nm) and macropores (pores >15 nm).
Catalyst durability, particularly as it relates to the retention of high power performance, is one of the major challenges facing the development of automotive fuel cell technology. Platinum or platinum-alloy particles lose electrochemical surface area during operation due to dissolution and subsequent Ostwald ripening and to particle migration and coalescence. Electrochemical oxidation of the carbon support enhances this particle migration and subsequent performance loss at high power. Oxidation of carbon support also causes the collapse of the electrode thickness and electrode porosity, hindering reactant transport and subsequent performance loss. Therefore, it is a common practice for those skilled in the art to avoid oxidation of carbon support.
On the other hand, in electrodes with small amount of Pt or low Pt surface area, large fuel cell performance loss is observed. This is due to the need to support larger flux of reactant oxygen or hydrogen to the Pt surface. This is particularly difficult for Pt particles that are embedded in carbon particle micropores. Accordingly, there is a need for improved catalyst layers.